Nucleating agents are employed as additives in polymer resin in the manufacture of plastic articles. Such manufacture may be by various methods, including by injection or extrusion molding.
Solid bicyclo[2.2.1]heptane dicarboxylate salt-containing thermoplastic nucleating additives are known. One commercially useful nucleating agent sold by Milliken & Company of Spartanburg, S.C. is Hyperform® HPN-68L. This nucleating agent is of the chemical structure below:

Hyperform® is useful in manufacturing plastic articles. It enables the polymer to achieve a significant cycle time reduction in the molding process compared to using non-nucleated polypropylene (“PP”). Cycle time reduction can be achieved in part by inducing in the resin a higher Tc (crystallization temperature), which enables the molten resin to harden at higher temperature. This decreases the necessary cooling time and facilitates ejection of the articles from the mold at a faster rate than would be possible without the use of a nucleating agent. A common defect in a process as injection molding is deformation of the molded part as a result of different stresses caused by polymer orientation. Longer mold residence time is needed to stiffen the material to such an extent that it can withstand these stresses and inhibit the deformation. Cycle time can thus also be reduced by optimizing the shrinkage properties of polypropylene by adjusting the orientation of the polymer crystals by a nucleating agent. Nucleating agents that adjust the shrinkage to be more isotropic, i.e. shrinkage in the machine direction becoming equal to shrinkage in the transverse direction are favorable for reducing cycle time where deformation is the limiting factor in the molding process. Cycle time is very important in the economic viability and profitability of molded part manufacture.
U.S. Pat. Nos. 6,465,551; 6,562,890; and 6,995,202 disclose dicarboxylate metal salt compositions.
The use of clarifying agents to reduce haze in articles manufactured from crystalline polyolefin resins is known. Some compounds may serve as both nucleating agents and clarifying agents. Representative acetals of sorbitol and xylitol, which have been employed as clarifying agents, are disclosed generally in the Rekers, U.S. Pat. No. 5,049,605, bis(3,4-dialkylbenzylidene). Among clarifiers based on diacetals of sorbitol and xylitol, Millad 3988® is a commercially successful clarifier for polypropylene. It is a dimethyl substituted dibenzylidene sorbitol (“DMDBS”), 1,3:2,4-bis (3,4 dimethyl benzylidene sorbitol). Millad 3988® is manufactured and distributed by Milliken & Company of Spartanburg, S.C.
Milliken & Company commercially sells a nucleating agent product known as Hyperform® HPN-20E which employs a calcium metal ion, this compound having the chemical structure:

Other known compounds useful for nucleation include sodium 2,2′-methylene-bis-(4,6-di-tert-butylphenyl) phosphate (from Asahi Denka Kogyo K.K., known commercially as NA-11®), as shown below:
Such compounds all impart high polyolefin crystallization temperatures; however, each also exhibits its own drawback for large-scale industrial applications. U.S. Pat. Nos. 4,463,113 and 5,342,868 disclose crystalline synthetic resin compositions of cyclic organophosphoric esters.
When a polymer is melted into a thermoplastic article, the force applied in shaping the article creates stress in the article. Polymers typically have long chains and long relaxation times, therefore residual stress exists in such plastic articles. The residual stress will release after the article is shaped which manifests as shrinkage of the article. Generally when a nucleating agent is added, because of short cooling time, the orientation and residual stress from molding process are maintained. Plastic articles with nucleating agents typically have higher shrinkage than articles without nucleating agent. Sometimes there is too much shrinkage and the plastic articles are out of dimensional specifications. To manufacture parts that meet the dimensional specifications, some companies have tried to decrease shrinkage by adding impact modifying compositions or talcum.
A nucleating agent for polypropylene with a combination of positive material properties, like high Tc, low crystallization half time (t1/2), isotropic shrinkage, and high stiffness would be highly desirable. Phosphate ester salts, like NA-11® and NA-21® (manufactured by Asahi Denka Kogyo Kabushiki Kaisha of Japan) are known to incur relatively high stiffness in injection molded articles. However, warpage caused by anisotropic shrinkage is often an undesired side effect of such materials. Such warpage is a disadvantage of using phosphate esters.
Each nucleating composition has its advantages and disadvantages. This has created a long-felt need in the polyolefin nucleator compound industry to provide compositions that minimize such problems and provide excellent peak crystallization temperatures for the target polyolefin. Unfortunately, it is a significant challenge to find nucleators exhibiting exceptionally high peak crystallization temperatures, low hygroscopicity, excellent thermal stability, high stiffness, and relatively low amounts of shrinkage or warpage in finished articles.
Blends of more than one nucleator used together have been tried, but are not always successful. Furthermore, this is a highly unpredictable area of the chemical arts, and there is usually no any way of knowing what will be effective until it is tried, and tested, and a relatively substantial amount of work is done.
U.S. Pat. No. 6,586,007 is directed to a combination of 3,4-dimethylbenzylidene sorbitol (DBS) and p-methylbenzylidene sorbitol (mDBS). U.S. Pat. Nos. 6,521,685 and 6,585,819 are directed to additives that comprise a blend of (a) bicyclic salts, and (b) benzylidene sorbitol acetals.
A commonly owned application assigned to Milliken and Company is U.S. Ser. No. 11/078,003 to Hanssen. This application is directed to a blended nucleating or clarifying composition for thermoplastics comprising a blend of more than one species of nucleating agent, the blend comprising: (a) a first nucleating agent of a carboxylic acid salt compound; and (b) a second nucleating agent of a phosphate-containing salt compound.
A nucleating agent additive composition that could achieve the remarkable advantage of cycle time reduction for molding processes and also exhibit little shrinkage as compared to current commercial systems would be very desirable, so that the nucleating agent could be easily used by thermoplastic manufacturers. This invention is directed to such a composition and methods for its use in the manufacture of polymeric articles. A nucleating agent composition that afford these advantages is desirable.